This invention pertains to the art of heat pumps generally, and in particular to the art of refrigerant flow circuiting and circuiting changes occurring when the heat pump is shifted between heating and cooling modes of operation.
While the invention is considered broadly applicable to heat pumps of various sizes and types, it will be described herein as embodied in roof-top packaged heat pumps of nominal 71/2 and 10 ton sizes. The general structural arrangement of a roof-top packaged unit which, as disclosed, was arranged for a cooling operation only but adaptable to be modified for operation as a heat pump is shown in U.S. Pat. No. 4,139,052. In modifying that particular type unit for heat pump operation, while simultaneously attempting to achieve high capacity per unit volume, high energy efficiency ratio in cooling, and high coefficient of performance in heating, several changes were required in connection with the indoor and outdoor heat exchangers or coils. The surface areas of both the indoor and outdoor coils are increased relative to the cooling-only unit to offset the deleterious effect of the added refrigerant pressure drop resulting from the addition of the required refrigerant reversing valve necessary for heat pump operation. The added pressure drop for refrigerant flowing through the suction side of the valve in effect reduces the available temperature difference between the airstream and the refrigerant, thus making it necessary to increase the effective surface areas of the coils. Of course, the increase in size of the outdoor coil results, in the cooling mode of operation, in a more effective condensing section. The net result of the increase in coil sizes is that in the cooling mode of the heat pump the system operates at approximately the same capacity over about the same net temperature difference as does such a unit designed for cooling only and which hence has less refrigerant pressure drop.
The genesis of this invention stems from these changes in the coils. The end result of the invention is to increase the effectiveness of the outdoor coil in a heat pump by providing adequate subcooling to the refrigerant when the heat pump is operating in a cooling mode, while also allowing that same coil to operate relatively efficiently as an evaporator in the heating mode.